Diode bridge for Valve Buffered GC

[falcott]

Hi there,
a couple of questions from a slow builder of a valve/tube buffered gainclone based on Joe Rasmussen's schematics.

I am using a regulated supply (LM317) for the filament 6.3v from a 9v torroid based on FranzG's schematic.
The valve itself is using 35v from a 25v torroid.

a.1) What is the present thinking about the type of SS diodes to use for valve power rectification? I was going to use 1N4007, but what about something softer, like Schottkys for example? Or 1N4937? Or the MUR types that are the GC builders' favourites?

a.2) Does the type of rectification really matter for the filament? If so, how does it affect the sound? (Ditto for the question above.)

b) For a slow startup, how and where does one fit a thermister in the supply to the valve? And what value should I use?

Thanks for your help!

[Eli Duttman]

Does the type of rectification really matter for the filament?

I believe it does. We endure great pains keeping SS diode switching noise out of B+ rails. Switching noise can get into the signal via the filament supply too. IMO, "noiseless" Schottky diodes are strongly indicated in filament supplies.

Your project is low voltage. Schottky diodes EVERYWHERE is (IMO) the way to go.

The big reason for soft starting B+ supplies is prevention of cathode stripping. Cathode stripping is not much of a problem when anode voltages are below 400 V. At the LOW voltage you are talking about, the phenomenon is non-existant. You don't need a thermistor.

[falcott]

Thanks very much for your advice!

I started this project 2 years ago (yes, and I am a slow renovator of houses too! ). Back then I emailed Joe Rasmussen about the idea of using Schottkys for the valve supply. (He had recommended 1N4007s.) He replied that he liked my idea, but didn't say why.

Isn't it nice to have confirmation?

[falcott]

Okay, I've got some MBR1100's, and they're now mounted on the board.

Just wondering: has anyone had any experience with any specific Schottky type in this sort of application? I would love to hear about it.

Thanks!